Non-volatile memory (“NVM”) refers to semiconductor memory which is able to continually store information even when the supply of electricity is removed from the device containing the NVM cell. NVM includes Mask Read-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), and Flash Memory. Non-volatile memory is extensively used in the semiconductor industry and is a class of memory developed to prevent loss of programmed data. Typically, non-volatile memory can be programmed, read and/or erased based on the device's end-use requirements, and the programmed data can be stored for a long period of time.
In general, NVM cells contain a charge-storage layer situated above a channel region of the cell. When appropriate programming voltages are applied to a source and a drain, which are positioned adjacent to opposite sides of the channel region, and to a gate situated above the charge-storage layer, charge carriers moving across the channel enter the charge-storage layer. In subsequent reading operations, the presence or absence of the trapped charge carriers can be detected based on the measured channel current.
Conventional floating gate flash memory cells, which are a type of NVM, generally employ a memory cell characterized by a vertical charge-storage stack of a first dielectric, a first conductive (charge-storage) layer over the first dielectric, a second interlevel dielectric over the first conductive layer, and a second conductive layer (control gate) over the interlevel dielectric. In floating gate memories, the charge is stored throughout the entire conductive storage layer. Thus, the memory states are trapped charge or no trapped charge, i.e., a single bit of data.
Another type of NVM cell that has received a great deal of attention in recent years based on increasing demands for higher memory capacity and smaller cell dimensions employs localized charge-storage to provide two bits of data per memory cell. In such devices, a non-conductive, charge-trapping layer is disposed between the channel and the gate, and preferably between two dielectric layers, such as silicon dioxide. One such type of NVM is referred to as nitride read only memory (“NROM”) and generally comprises an oxide/nitride/oxide (“ONO”) charge-trapping structure. One primary advantage of NVM cells which employ charge-trapping materials is the localized storage of charge which allows two bits of data to be stored in each cell. Such localized charge storage allows one charge (bit-1) to be stored in the charge-trapping layer in an area proximate to one source/drain region and another charge (bit-2) to be stored in the charge-trapping layer in an area proximate to the other source/drain region.
Unfortunately, non-volatile memory cells which employ charge-trapping layers and store charge in a localized manner are not without problems. For example, in nitride storage memory cells where the charge-trapping layer generally comprises a silicon nitride layer sandwiched between two silicon dioxide layers (“ONO structure”), hydrogen atoms may become trapped at the interfacial sites between the silicon substrate and the bottom oxide (first insulating) layer of the charge-storage ONO structure. Hydrogen can be generated and introduced into the charge-storage structure via various techniques used for forming one or more of the layers, for example, via chemical vapor deposition techniques. Although hydrogen can passivate dangling bonds at oxide/silicon interfaces, silicon-hydrogen bonds can be weakened when devices are subjected to hot-hole injection, which can be used for erasing various kinds of non-volatile memories, including ONO structures. Therefore, it is believed that interfacially trapped hydrogen is at least partly responsible for some loss of threshold voltage in memory devices having an interface between silicon and an oxide.
Accordingly, there is a need in the art for non-volatile memory cells with localized charge-storage having improved properties and which suffer less from the problems associated with interfacially trapped hydrogen.